Feather corticosterone and stable isotopes explain fledging and adult breeding success in a burrow-nesting seabird, the Yelkouan shearwater Puffinus Yelkouan

Abstract Seabirds face a trade-off between offspring provisioning and foraging effort. The hormone corticosterone regulates energy balance, while stable isotopes are proxies for diet composition. Measurements of Mercury (Hg), corticosterone (CORT f ) and stable isotope values of C (δ 13 C) and N (δ 15 N), integrated during feather growth in Yelkouan shearwater ( Puffinus yelkouan ) nestlings, were used to understand whether chick provisioning explains fledging and adult breeding success. Chicks at colonies and fledglings that failed their first fledging attempt were sampled in three breeding seasons (2020–2022) on Malta (36.01° N, 14.35° E). Failed fledglings were found at sea unable to fly or on urban coasts, presumably attracted by light pollution. Adult shearwaters were GPS-tracked in multiple seasons (2012–2022). Differences in provisioning measures (δ 15 N, δ 13 C and CORT f ) between failed fledglings and chicks at colonies, associations between provisioning and adult breeding success, and adult foraging strategies were investigated. Shearwater nestlings showed a response to variations in diet, by which CORT f was inversely related to δ 15 N and δ 13 C. Hg load was larger at higher trophic position, while there was no evidence for CORT f suppression by Hg. Failed fledglings had disproportionally higher CORT f , indicating that rearing conditions affect fledging success. Adult breeding success was related to provisioning in nestlings, while adult shearwaters at the colony with higher breeding success made shorter foraging trips. Findings suggest that several chicks experience sub-optimal provisioning, with negative implications on fledging and grounding risk during light pollution attraction. Foraging conditions affect adult breeding success, specifying that seabird conservation at colonies needs complementary marine restoration.